config: fix a type error in pyright due to different narrowing rules

For some reason pyright infers the type of `essential_plugins` and
`default_plugins` as `tuple[Literal[...], ...]` even if they are
explicitly annotated as `tuple[str, ...]`. This then causes an error in
the `builtin_plugins.add` calls.

Replace with one-shot initialization appeases pyright and is nicer
regardless.

Author: bluetech

src/_pytest/config/__init__.py

@@ -273,9 +273,11 @@ def directory_arg(path: str, optname: str) -> str:
     "faulthandler",
 )
 
-builtin_plugins = set(default_plugins)
-builtin_plugins.add("pytester")
-builtin_plugins.add("pytester_assertions")
+builtin_plugins = {
+    *default_plugins,
+    "pytester",
+    "pytester_assertions",
+}
 
 
 def get_config(